add checkpoint stats of snapshot and mapping files of pg_logical dir

Hi,

At times, there can be many snapshot and mapping files under
pg_logical dir that the checkpoint might have to delete/fsync based on
the cutoff LSN which can increase the checkpoint time. The
user/admin/developer might want to get an answer for "what's the total
time spent in deleting these files or what's the total size of the
files deleted or what's the cutoff LSN with which these files were
deleted". How about adding these stats to existing checkpoint stats?
IMO this information will be super useful. Attaching a patch doing
that. The output looks like [1].

Thoughts?

[1]
2021-10-29 18:48:10.731 UTC [615566] LOG: checkpoint complete: wrote
449 buffers (2.7%); 0 WAL file(s) added, 0 removed, 0 recycled;
write=0.008 s, sync=0.035 s, total=0.081 s; sync files=2,
longest=0.033 s, average=0.018 s; distance=6338 kB, estimate=6338 kB;
snapbuild snapshot file(s) removed=4, size=516 bytes, time=0.001 s,
cutoff LSN=0/14C8E08; logical rewrite mapping file(s)
removed/synced=0, size=0 bytes, time=0.001 s, cutoff LSN=0/14C8E08

Regards,
Bharath Rupireddy.